The present invention relates to a fuel cell system and a method, and more particularly, to a fuel cell system and a related control method satisfying a water balance and a heat balance of the fuel cell system while enabling a fuel cell to be efficiently operated.
Fuel cell systems are well known and are supplied with hydrogen and oxygen gases that react to convert chemical energy directly into electrical energy.
In a fuel cell system having a fuel cell utilizing an electrolyte layer, the fuel cell generally includes an anode electrode and a cathode electrode separated by the electrolyte layer. A reactant such as fuel gas containing hydrogen is supplied to the anode electrode, and an oxidant such as oxygen containing gas is supplied to the cathode electrode. In such a fuel cell utilizing the electrode pair, electrochemical reaction occurs at surfaces of the electrolyte as expressed by chemical formulae (1) and (2) to provide electrical energy.Anode reaction: H2→2H++2e−  (1)Cathode reaction: 2H++2e−+(½)O2→H2O  (2)
In such fuel cell systems, various methods of supplying fuel gas to the anode electrode are known to include a technology to directly supply hydrogen gas from a hydrogen storage reservoir or a technology to reform hydrogen containing fuel for subsequently supplying resulting hydrogen containing gas. The hydrogen storage reservoir includes a high pressure gas tank, a liquefied hydrogen tank and a hydrogen occlusion alloy tank. Hydrogen containing fuel includes natural gas, methanol and gasoline. Oxygen containing gas to be supplied to the cathode electrode is generally comprised of air.
With such fuel cell systems, in order for the fuel cell to effectively utilize an electrolyte performance to enhance an electric power generating efficiency, the electrolyte layer must be maintained at an optimum humidified condition. To this end, there are some instances where the fuel cell includes s structure to humidify fuel gas and air to be introduced to the fuel cell.
Japanese Patent Application Laid-Open Publication No. H8-250130 discloses a fuel cell adapted to utilize pure water as cooling medium to humidify an electrolyte layer via a porous type bipolar plate.
When reforming fuel gas to produce hydrogen, further, there are some instances where water is utilized to be reformed. For example, when reforming methanol to produce hydrogen, hydrogen is obtained in a chemical formula (3).CH3OH+H2O→3H2+CO2  (3)
Thus, in order to apply the fuel cell to an automotive vehicle, an efficient water balance must be appropriately maintained within the fuel cell or the fuel cell system involving the reformer because of a variety of factors. For example, water lost from the fuel cell to cause a shortage of water may be replaced by pure water supplied to the fuel cell at regular intervals, resulting in a troublesome maintenance work and a serious deterioration in a serviceability of a fuel cell powered vehicle.
On the other hand, when operating the fuel cell, it produces heat. To this end, the fuel cell system includes a structure wherein coolant such as water is supplied to the fuel cell, with coolant being introduced to a cooling device utilizing a heat exchanger such as a radiator to radiate heat outside from the fuel cell. A heat amount being radiated outside the vehicle at this instant is subjected to various factors such as an atmospheric temperature and a vehicle speed.
Under such circumstances, if the generated heat amount of the fuel cell exceeds a heat amount to be radiated by the cooling device, then the temperature of the fuel cell raises, with a resultant difficulty in continuously operating the same. Thus, the fuel cell system is required to have a heat balance suitably maintained at an appropriate level.
Japanese Patent Application Lai-Open Publication No. H10-172599 discloses a technology to establish a water balance of a fuel cell system through a control of an operating pressure of a fuel cell. More particularly, in a case where the atmospheric temperature increases to cause the amount of water contained in exhaust gases discharged from the fuel cell system to increase with a resultant difficulty in maintaining an efficient water balance of an overall structure of the system, a control is performed for increasing the operating pressure of the fuel cell to reduce the amount of water discharged from the fuel cell.